A proximity sensor is a sensor that is able to sense the presence of a nearby object without requiring any physical contact with that object. An object to be sensed by a proximity sensor is typically referred to as a target. In one example, an electronic device may comprise a proximity sensor that is able to sense the presence of a user's skin, and the electronic device may be configured to adjust certain behaviours or processes dependent on an output of the proximity sensor. For example, where the electronic device is a mobile telephone with a touch screen, the proximity sensor may be positioned such that it is able to sense the presence of the user's skin when the user is making a voice call (i.e., when the mobile telephone is oriented with the speaker close to the user's ear, and the microphone close to the user's mouth). Responsive to the proximity sensor sensing the presence of the user's skin, the mobile telephone may be configured to deactivate the touch screen. In this manner, the proximity sensor may be used to conserve power and to prevent inadvertent activation of the touch screen during a voice call.
A proximity sensor typically comprises an emitter and a detector. The emitter emits electromagnetic radiation or light, such as infrared (IR) light, over a region which will herein be referred to as the emitter field of view (FOV). The detector detects light over a region which will herein be referred to as the detector FOV. When light emitted by the emitter is incident on a target, the light may be reflected by the target in such a manner that the target-reflected light is incident on the detector within the detector FOV, and therefore able to be detected.
There may exist a region close to the proximity sensor where the emitter FOV does not overlap with the detector FOV. This region will herein be referred to as the blind zone. Depending on the specific emitter and detector used, as well as their optical configuration within the electronic device, it may be impossible for the proximity sensor to sense a target that is located within the blind zone.
Ideally, the emitter and the detector would be isolated from one another such that the only light detectable by the detector is light external to the proximity sensor (e.g., light reflected from a target and any ambient light external to the electronic device comprising the proximity sensor). However, in practice, the detector also detects some of the light, emitted by the emitter, that has not reached the exterior of the electronic device. This undesirable contribution will herein be referred to as crosstalk. Crosstalk arises when light emitted by the emitter is internally reflected off of various surfaces within the electronic device and is detected by the detector without having ever left the electronic device. Crosstalk contributes to noise. More crosstalk results in a proximity sensor with a lower signal to noise ratio.
Sometimes the intensity of the light reflected off of a target is too low, relative to the noise, to be detected by the detector. This may occur, for example, when the target is too far away from the proximity sensor, or when the target comprises a material having a low reflectance. For example, the reflectance of light off of black felt or black hair may be lower than the reflectance of light off of other targets. If the intensity of the reflected light is too low, relative to the noise, then the proximity sensor may be unable to sense the presence of the target. In addition, as described previously, when the target is too close to the proximity sensor, the target may be located within the blind zone such that the presence of the target goes undetected by the proximity sensor. These and other limitations of the proximity sensor may depend on the specific emitter and detector used, as well as their optical configuration within the electronic device.